The present invention relates generally to a method for coating a liquid composition onto a substrate surface to form a coating thereon comprising one or more layers. More particularly, the present invention relates to a method for formulating one or more the liquid compositions forming such coating layers, which maximizes the resistance of such compositions to variations in thickness of the layers formed therewith.
It is well known to coat a moving web or substrate with a composite layer of liquid coating composition. The composite layer may comprise one or more layers simultaneously coated onto the moving web. Coating can be performed by a variety of methods including, for example, curtain coating, slide bead coating, and extrusion hopper coating with more than one slot. In the manufacture of various types of coated web substrates, a serious problem can arise as a result of variations in thickness of the coated composition in the direction transverse to the coating direction (the direction of travel of the web through the coating apparatus). Such variations are referred to in the art as xe2x80x9crunning streaks.xe2x80x9d In some coating applications, for example, in the manufacture of photographic films and papers, relatively slight variations in thickness, on the order of 1% or less, can render the coating unacceptable.
Such streaks may be formed, from a variety of well-known causes, at any point where the coated composition is still liquid. For example, composition may be caused to move laterally, creating a local increase in composition thickness and leaving a corresponding local decrease in thickness. Thus the practical effect of lateral flow is the sum of the thickness increase and decrease.
Many photographic products are manufactured by a coating technique known in the art as xe2x80x9ccurtainxe2x80x9d coating, wherein liquid composition (also referred to herein as xe2x80x9cemulsionxe2x80x9d) is extruded from a coating die having a linear coating lip and falls free as a liquid sheet or curtain under gravity onto a substrate passing beneath the die, where it forms a coated layer or layers on the substrate. As is well known in the coating art, the curtain is vulnerable to deformation by air currents impinging on the curtain. Many schemes are known in the art for mechanically shielding the curtain, such as providing close-fitting screens on either side of the curtain or enclosing the die and curtain in a stagnant air chamber. See, for example, U.S. Pat. No. 5,114,759 to Finnicum, et al, U.S. Pat. No. 4,287,240 to O""Connor, and U.S. Pat. No. 5,976,630 to Korokeyi, et al. None of these schemes can be totally successful because of practical considerations such as turbulence caused by entry and exit of the substrate to the chamber and condensation, which can drip from chamber surfaces onto the composition. Further, it has been shown that even very low velocity air currents, on the order of 15 feet per minute or less, can cause unacceptable curtain deformation.
A complementary approach to mechanical shielding is to add surfactant to compositions to be coated to increase resistance to thickness deformation caused by flow of the coating on the web. Many compositions consist of multiple individual layer-forming compositions (referred to herein as xe2x80x9clayersxe2x80x9d even before the actual point of coating onto the substrate) delivered simultaneously as a coating pack or composite layer from a multiple slot coating die; thus layer compositions exposed to air in a falling curtain may differ between the front side and the back side of the curtain, and each such layer may include surfactant to optimize resistance of the overall coating pack to streak formation. See, for example, U.S. Pat. No. 5,773,204 toBaumlin.
For the purpose of immobilizing (preventing flow of the coating on the web ) and solidifying the coated layers after the coating point, independently of the means used to apply these layers to the support, the coated layers are subjected to air currents that either set (which increases viscosity) and dry them, or simply dry them. The immobilization process typically is done over a period of seconds, during which the coating is subjected to the impact of the air currents. When these currents indirectly impinge on the coated layers, they can lead to thickness variations in the coated layers in the way of a random blotchy pattern referred to in the art as xe2x80x9cmottlexe2x80x9d. More severe thickness variations can be caused by air currents impinging directly on the coated layers, such as in the form of impinging air jets typically used to produce substantially higher heat transfer rates and accelerate the immobilization rate. The corresponding thickness variations appear as straight lines of some width, which are known in the art as xe2x80x9cstreaksxe2x80x9d.
A difficulty in the art of formulating compositions for coating is determining the optimum concentration of surfactant. Presently, the amount selected is determined empirically by trial and error on representative product layers on a pilot coating machine, or with real product layers on a production coating machine. This approach is known to be very time consuming and costly, especially with regard to the generation of waste or sub-optimal coatings.
Thus, there is a need for a method for simple, off-line determination of the optimum level of surfactant for a coating composition.
It is therefore an object of the present invention to provide an improved method for predicting the optimal concentration of surfactant for a coating composition.
It is a further object of the invention to provide an improved method wherein simple, inexpensive off-line determinations may be correlated with pilot-scale trial coatings to predict streak or mottle-resistance optima.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by measuring the dynamic surface tension (DST) of a proposed liquid coating composition over a range of surfactant levels to determine the surfactant concentration that produces the maximum surface tension gradient. Measurements are made by a method related to the Wilhelmy Blade Method, in which a surface of a static pool of composition to be measured is placed in contact with the lower edge of a suspended blade, and the force required to lift the blade from the surface is determined. The static method is modified such that the surface of the composition touching the blade is continually refreshed, to simulate the formation of fresh curtain surface, by pumping the composition upwards through an open cylinder and allowing the composition to spill over the edges (xe2x80x9coverflowing weirxe2x80x9d). The bulk surfactant concentration providing maximum resistance to coating streakiness and mottle is highly correlated with the concentration providing maximum surface tension gradients in the overflowing weir. Thus, for new or non-optimized air-contact layers, the optimum surfactant concentration can be predicted quickly and inexpensively through off-line measurement of surface tension using an overflowing weir technique. The technique is suitable for optimizing compositions comprising each of the outer layers of a multiple-layer coating pack in a curtain coating operation, as well as for those compositions comprising the top layer of a multi-layer composite coated in a bead or curtain coating operation. Although much of the method was developed in the context of the curtain itself in a curtain coating operation, surprisingly, it has been found that the method yields a good result for coating processes in general, including, for example, curtain coating, slide bead coating, and extrusion or slot coating, regardless of where the air disturbance occurs. The disturbance may be on the slide surface, in the curtain, or on the web in chill setting or early drying of the coating. In addition, it has been found that the method can also be applied to non-aqueous coatings. Further optimization for specific coating operations and coating formulae can be optimized empirically by those skilled in the art.